Radio receiver



Feb. 25, 1947. DL D GRIG 2,416,308

RADIO RECEIVER VAL 0E INVENTOR. 00A/HL@ G77/E6' Feb. 25, 1947. D. D. GRIEG 2,416,308

.RADIO vREGEIVER Filed July 2o, 1943 zsheets-sheet .2

BY wm mv Yv Patented Feb. 25,1947

' UNITEo .STATES PATENT r.2,416,308 Y' ymimo RECEIVER l Donald D. Grieg, Forest Hills, `N. Y., assignor t Federal Telephone and Radio` Corporation,

Newark, -N. J., 'a corporation of ADelaware Y,

Application Jly'zo, 1943, serial No. 495,419

1 6 oiaims.- (01.250-20) This invention relates to radio receiving circuits and more particularly to` electricall circuits for translation of pulse energy into impulses of greater intensity.

In the copending application of E. Labin and myself, Serial No. 488,181,7led May 24, `1943are certain forms of triggerable*oscillating circuits tunable for response to a given pulse source such as a carrier #wave of time modulated pulses for translation of the carrier pulsesinto impulses of greater intensity. The oscillating circuits therein disclosed are adjusted for normal intreval operation at a frequencyslightly lower than the unmcdulated pulse repetition rate of the pulse source. The pulses of the source lcan then con; trol the oscillation intervals of' thev circuit when the circuit operation is brought into approximate synchronization'with the pulse source.,

While this adjustment of the (oscillating circuit may be madeto eliminate most noise interference, other interference pulses `s uch as -produced for jamming purposes can trigger the circuit to operation and effect a translation of unwanted pulses and the consequent elimination of at least a percentage of the wanted signal pulses.

In a further copending application of E. Labin and myself, Serial No. 488,183, led lMay 24, 1943,l a radio receiving'circuit is disclosed Which'includes an oscillating trigger circuit provided with a multivibrator for generating rectangular voltage pulses which vare applied to the oscillating trigger circuit to increase the susceptibility of its operation to input pulses during the selected width of the multivibrator pulse. The multivibrator is adjusted for operation at a frequency corresponding to the unmodulated pulse repetition rate of the pulse source and the width of the rectangular pulse is selected so that each rectangular pulse will cover an interval equal to at least the maximum degree of modulation ap-V plied to the pulses at said source. The applied multivibrator pulse increases sharply the susceptibility of the circuit and maintains this increased susceptibility throughout the duration of the rectangular pulse so that regardless' of the degree of'modulation of the input'pulses, the circuit will be susceptible thereto. The susceptibility of the ,circuit during the interval between successive multivibrator pulses is lewin comparison and the circuit during such intervals normally Will not respond' to input pulses unless the latter are of exceptionally high intensity.

It is'an object of my present invention to provide another improved radio receiving circuit for translation and demodulation of time modulated pulses wherein the translation involves the generation of a sinusoidal voltage which is uti.

lized to render the oscillating trigger circuit hi'ghly'susceptible to pulses occurring vonly dur-v ing an interval approximately equal to the maximum degree of modulation applied tothe pulses of the lsource the reception lof which'isdesi're'd".

Another object of the inventionis to' 'provide a method and means for translation `of 'desired pulse energy into impulses of rifiuchgreater'in#y tensity'lwhile eliminating t a'hlgh'` degree noise and otherunwanted pulses.A f jj One feature ofmy present invention,`for example; -includes anfoscillatin'g trigger circuitfin combination with a shoc'kexcitable 'resonant cir-f cuit which is provided withja damping'tube whichmay be controlled in such a manner as to damp out the oscillations "set up inthe ,reso-4 nant circuit the instant a trainiof oscillations has served its purpose.` soidaljvoltage established by operation of the resonant circuit is applied tothe ltrigger oscillating circuit so` as to control: the susceptibility thereof in a mannerto increase the susceptibility sharply for the interval the Wanted signa1 pulses are expected. This control of the susceptibility ofthe oscillating trigger circuit takes placelat a frequency which corresponds with" the unmod-l ulated pulse repetition rate of thev desiredpulse source. `When a Wanted 'pulse operates" the oscillating trigger circuit, thezdampingF feature of the resonant circuit responds to damp 'the-'exa isting oscillations of the'resonant circuit and thereby conditi'onthe circuit-for shock excitation by the termination ofthe operation' 'of the" oscil lating trigger circuit. The termination off-the oscillating trigger circuit 'shock excites the. resonant circuit to oscillation which thereafter con` trols the -susceptibility ofthe oscillating trigger circuit according to the tuning of the resonant circuit.

.'The above and other objects and featuresfof my invention will become more apparent upon reference to the following detailed description' when read in connection with ythe vaccompanying drawings, in which: I Fig. 1 is a schematic wiring diagram o'f a radio receiving'circuit according to my invention;4

steps performed by' the receiverof Fig'. "1"; f.

Figs. 3 and 4 are schematic wiring diagrams'of two additional embodiments Afor'perfo1"ming thesinusoidal voltage generating-anddamping 'fea- -f Fig.2is1a graphical illustration-of theopeif'ating.

r tures of my invention; and

Fig. 5 is a graphical illustration showing curves to assist in the explanation of the embodiment: shown in Fig. 4.

Referring to Fig. 1, a T. M. (time modulationl" radio receiver isshown having the. usual antenna.,`

In "other words, the sinul- R1 and R2.

; the modulating signal.

3 I0, an oscillating trigger circuit I2,ra local sinusvoidal Voltage generator yI4 and a T. M. demodulator I8. The oscillating trigger circuit I2 is provided with a, tunable 'inductance-capacitance circuit'LiCi for detection of'R. F. energy of any particular carrier Wave obtainable from an an-` tenna coupling coil I9. The oscillating circuit I2 includes a vacuum tube having an anode 2i, a control grid 22,v a cathode 23 and a screen grid 2-4. One side 25 of the circuit L1C1 is connected through a blocking condenser 26 tothe grid 22; The opposite side 2l of the circuit L1C1 is the bias condition of the damping tube 40. Curve f represents the sinusoidal voltage output of the voltage generator I4 which is applied to the grid 22 of the oscillating trigger circuit I2. i

, Assuming that no sinusoidal voltage is applied to thegrid 22 ofthe circuit I2, the negative grid voltage may be represented as Vdecreasing (be- :oming less negative) gradually as indicated by lthe portion 'I0 of curve b. The normal-period of operation of the circuit I2 in the absence of signal reception is determined according to selected `time constants of the grid circuit (con-` grounded. For triggering action, 'the cathode 23 i is connected to the coil L1. The sc'reengridf24 is connected to ground through a condenser 3i) fior by-passing the radio frequency energy. The screen grid is also connected through a resistor 32 to a source of positive energy from battery terminal 34. Y i The anode circuit of the tube 20 includes 'the local sinusoidal voltage Ygenerator I4. The' generator I4 includes a resonant circuit comprising inductance coilLz and condenser C2 together with a damping `tube 40 the anode 4I and cathode 42v of which are connected across the resonant cir` cuit. The input 35 of the resonant circuit LzCz is connected to the anode 2I of tube 26. lThe op-` posite side of the resonant circuit isconnected1 q through resistors R1 and Rz to a source of positive energy lat terminal 34.-: A dierentiator circuit comprising a condenser Ca and a resistor Rs is connected by line 48 to a pointbetween'resistors A battery 50 is interposed between the resistor R3 and the side 45-.of the resonantv circuit. The outputconnection 52 from the diery entiator Yis applied tothe control grid 43 ofthe Vdamping tube' 4o. The sinusoidali vonage established inthe resonant circuit L2C2 is'applied to .Y y,

the grid 2-2of tube 2i) through coupling connection V54 and voltage dividersl R4 and'Rs. The impulse output ofthe oscillating trigger circuit is taken oil' from the line 48 thereby providing output voltl age pulses from across the resistor R2 which are applied through connection 56 to the T; M. de-

, modulator I8.

In Fig. 2, curve a shows a train of three signal pulses havingv an unmodulated pulsefrepetition rate represented by the period Ts. Thevpulses are shown' to be double modulated. In "double pulse modulationthe duration of every pulse is i usually constant and small compared with thev time interval between adjacent pulses and the pulses occur in ypairs with the time interval therebetween being proportionalto the amplitude of Pulses 6I and 62 comprise a pair of pulses which are modulated to one broken lines 63 and 64. Pulse 65 is the first extreme degree of displacement, the opposite dei gree of displacement being indicated by the y i y Curve b of Fig. 2 represents, generally, the grid voltageof the grid 22 during the reception of the signal pulses of curve a; Curve c represents the effective voltage impulse acting across the anode circuit of `tubec'ZIl during the oscillating operation 3 of the circuit I2. Curve d represents the output voltage impulses acrossresistor R1 with respect to point 45 in response to the impulses of curve c. The impulses of curve d are Vapplied to the dinerfentiator Ca, Ra and to the T. M. demodulator I8. I Curve e represents the output of the diiierentiator circuit C3, R3 together with an indication of the polarity' relationship of the pulses with regard to denser 26 and resistors R4, R5) andthe biasing` potential applied to the screen grid 24. This normal vperiod' is represented approximately as the period To. The reception of pulse 6! triggers the oscillating circuit I2 into operation therebypro-f rangement Ca, R3 to produce positive pulse 'I3 andY negative Vpulse 14, curve e, the pulse 'I3 corresponding to the leading edge of the pulse I2 and the pulse 'I4 corresponding to the trailing edge thereof. The positive pulse 'I3 is of an amplitude suil'icient to overcome the negative grid bias applied by battery 50 on the grid 43 of the tube 40. This pulse energy renders the .tube conductive thereby reducing momentarily the .Q of the resonant circuit LzCz to such a value as to absorb any oscillatory energy present in thev circuit. This absorption of the oscillatory energy is such as to substantially completely da-mp out all oscillations occurring in the circuit including the shock excitation energy applied thereto by the leading edge of the impulse 1I, thereby resultingv in a zero potential in the coupling connection 54. The importance of this damping feature Will become more apparent as the description proceeds.

`The trailing edge 8I of the current impulse 'II operates to shockexcite the resonant circuit LzCz into-renewed oscillation (portion 82 of curve f) the frequency of which is determined by the tuning adjustment of the condenser C2. This sinusoidal voltage 82 is applied over connection 54 as hereinbefore described to the grid 22 of tube.

`The signal pulse 62 occurring during interval 85 adds to the peak part 84 of the grid potential Athereby causing it to pass through critical level 86 and initiate a period of oscillations in the circuit I2. As hereinbefore described, this oscillatingoperation produces an eiective current irn- 1 s pulse 9i, curve c, and by differentiating the corresponding voltage impulse 92r twopulses 93 and 94 are produced corresponding respectively to the Aleading and trailing edges of the current impulse QI. The positive pulse 93 produces conduction in the tube 40 thereby damping the sinusoidal voltage 82 to Zero potential as indicated at 95. The trailing edge of the current pulse 9| shock -signal pulses.

From the foregoing description it will be clear -that the damping feature operatesto discontinue :the sinusoidal voltage as soon as it has served its purpose. That is to say, the sinusoidal voltage 'is removed from the grid 22 as soon as the tubes and 40 are rendered conductive so that no :sinusoidal voltage will continue to exist following the sensitizing function for which it is supplied. Should the sinusoidal voltage be permitted to continue as indicated by the broken line 91 `oi? curve'f, it would adversely affect both the amplitude and timing of the next succeeding excitation of the circuit L2C2. Each sinusoidal wave 82, 96, etc.- is independent of the others so that 'substantially no variation of amplitude or timing lined by theportions 'I0 and.'|0a. By properly tuning the resonant circuit LzCz, the peak portion of a selected undulation of the sinusoidal voltage may be caused to cover a signal reception interval such as the interval 85 which correspond to the maximum degree of modulation of the signal pulses so that regardless of the degree of modulation of any particular signalipulse the circuit I2 will be rendered highly susceptible thereto. It is thereforeclear that by applying the sinusoidal voltage tothe control or other grid of tube 20, the oscillating circuit I2 is rendered highly susceptible during the intervals when siglnal pulses lfrom a given source are received and maintained less susceptible to input pulses for substantially all of the timeoccurring between the reception intervals of successive signal pulses.

The voltage output impulses across'resistorRi are applied to the T. M. demodulator |8 for demodulation in the usual manner. If desired, the vpulse energy may be differentiated to sharpen the pulse shape thereof before it is applied to the demodulator. The demodulator I8 may be of any known construction and preferably is of the character disclosed in my copending application Serial No. 459,959, filed September 28, 1942. The demodulation of the pulses according to the invention of the aforesaid application translates the. intelligence represented by the time displacements of pairs of pulses into corresponding amplitude variation which may4 then be audibly detected in known manner. w

In Fig. 3, I show a variation of the local sinusoidal voltage generator wherein the current input impulses |0I applied to the resonant circuit |00 are of positive polarity. This impulse energy may be taken from the cathode 23 of the oscillating circuit I2. The current impulses I0| are applied through the resistor R4 tothe resonant circuit |00. Output voltage impulses are taken from across the resistor R4 by connection |03 for application to the demodulator and also tolthe differentiator combination CsRs. The rst pulse of the differentiating operation is applied to the control'igrid |05 of the damping tube |06. The cathode and anode terminals of the tube |06 are connected across the resonant circuit` |00 so that when conduction is produced therein bythe differentiator, any oscillating energy present' in the circuit |00 is damped out. The resulting sinusoidal energy lof the circuit |00, which is similar to that of curve z', Fig. 5, is applied over connection 54a to the grid 22 of the oscillating circuit I2, Fig. l, similarly as hereinbefore described to obtain a desired control of the susceptibility of the oscillating circuit.

In Fig. 4, I show a further sinusoidal wave generator in which the damping period is prolonged for substantially the duration of the impulse applied thereto frcm the oscillating circuit I2. As in Fig. 3 the connections are so selected as to apply positive current impulses |0| through resistor R4 to the resonant circuit |00. The differentiator combination, however, is omitted in this embodiment. A battery I I0 is provided to normally furnish a negative bias on the grid |05 of the damping tube I 05. The current impulses IOI produce corresponding positive voltage impulses I I2 across the resistor R4 for application to the grid `|05 to overcome the negative bias provided by battery IIO and thereby render the tube conductive for the duration of each of the impulses II2.

In Fig. 5, the voltage impulses I|2 of curver h are shown in their relation with the resulting sinusoidal voltage of curve i in accordance with operation of the generator of Fig. 4. Assuming that the sinusoidal voltage ||5 exists upon-the reception of an actuating pulse, the passage of current through the tube 20 of the oscillating circuit I2, Fig. 1, will produce, when applied vto the circuit of Fig. 4, a" voltage impulse II2al. Since this impulse overcomes the bias on the tube |06, the tube |06 conducts thereby damping out the oscillations in circuit |00. The damping period continues for the duration of the impulse |I2a throughout the zero potential portion ||6 The trailing edge I I3 of the corresponding current impulse, however, operates to shock excite the circuit |00 substantially simultaneously with the termination of the conduction through tube |06. This simultaneous timing of the shock excitation of circuit |00 and the termination of tube conduction Vcauses some dissipation of the shock exciting energy, -but not so much as'to prevent renewed excitation of the circuit |00. This renewed sinusoidal voltage II'I is applied in the manner hereinbefore described to the grid 22 of tube 20 of Fig. l. The advantage of this embodiment over the preceding embodiments is its simplicity andk alsothe fact that the damping con duction of tube |06 is prolonged. It will be noted that the sinusoidal wave portions of curve i are initiated in a direction opposite to the sinusoidal portions of curve f of Fig'. 2. The number of undulations for each wave portion depends, of course, upon the tuning ofthe resonant circuit |00 and if desired the wave portion may jtake the form indicated at |20. While this latter form |20 lowers the susceptibility of the circuit I2 between signal reception intervals it is less sharp in its, control from the low sus. ceptibility level ito the high susceptibility level.

While I have shown the principles of my invention iny connection with specific apparatus, I recognize that many additional variations and ernbodiments may be devised without departing from the invention. It is to be understood, therefore; that the embodiments hereinshown and described are given by` way of example `only and notfas limiting4 the scope'of the invention as set forth in the objects and the appended claims..

l. In a radio receiver for time modulated pulse reception, an oscillating trigger circuitl operating at a lower pulse frequency than the unmodulated pulse repetition rate of a given time modulated pulse source for production of an impulse for each operating interval thereof, means to generate a sinusoidal voltage having a selectable frequency relationship with the unmodulated repetition rate of said pulses, means to apply the sinusoidal voltage to said trigger circuit to control the susceptibility thereof to actuation by input pulses, said frequency relationship -being selected to time an undulation of said sinusoidal voltage so that the peak portion thereof covers an interval at least as great as the maximum degree of modulation of the pulses from said source, means for .causing the initiation of the generation of said sinusoidal voltage upon termination of a cycle of trigger circuit operation, means to suppress the sinusoidal voltage prior to the initiation o circuit operation, 1

2. The radio receiver defined in claim 1 Wherei the sinusoidal voltage generating means includes a resonant circuit shock excitable in response to operation of said trigger circuit.

3. The radio receiver dened in claim 1 wherein the sinusoidal voltage generating means includes a resonant circuit shock excitable upon termination of a trigger circuit operation, and the means for suppressing the sinusoidal voltage includes means responsive to current ow in said trigger circuit during operation thereof to decrease the Q of the resonant circuit. Y

4. The radio receiver dened in claim 1 wherein the sinusoidal voltage generating means includes a resonant circuit and means for differentiating the impulse output of said trigger circuit to produce two pulses, the irstl of said pulses corresponding to the leading edge of said impulse and the second of said pulses corresponding to the pressing the sinusoidal voltage being responsive to said rst pulse to damp out any oscillations present in said resonant circuit, and said resonant circuit being responsive to said second pulse .to renew oscillations therein.

5. A radio receiver for time modulated pulse reception comprising an oscillating trigger circuit operating'at a lower pulse frequency than the unmodulated pulse repetition rate of a given time modulated pulse source for production .of an impulse for each operating interval thereof, means to produce upon .termination of said circuit operation a sinusoidal voltage having a selectable frequency relationship with the unmodulated repetition rate of said pulses, means to apply the sinusoidal voltage to said circuit to control the 'susceptibility'thereof to actuation by input pulses, said frequency relationship being selected to time an undulation of said sinusoidal voltage so that the peak portion thereof covers an interval at least as great as the maximum degree of moduA Vthe time modulated pulses of a given source into an equivalenttrain .of time modulated impulses Atrailing edge of said impulse, the means for sup- 'comprising an oscillating trigger circuit operating at a lower pulse frequency than the unmodulated pulse repetition rate of said given source for production of an impulse for each operating interval thereof, means to generate a sinusoidal voltage having a selectable frequency relationship with the unmodulated repetition rate -of said pulses, means to apply the sinusoidal voltage to said trigger circuit to control the -susceptibility thereof to actuation by input pulses, said frequency relationship being Aselected to time an undulation of said sinusoidal voltage so that the peak portion thereof ycovers an interval corresponding in time to the maximum degree of modulation of theV pulses from said source, means for causing the initiation of the generation of said sinusoidal voltage upon termination of a cycle of trigger circuit operation and means to suppress the sinusoidal voltage following the initiation of a vcircuit operation.

7. The pulse translator defined in claim V6 wherein the sinusoidal voltage generating means includes a resonant circuit shock excitable in response to operation of vsaid trigger circuit.

8. The pulse translator defined in claim 6 wherein the sinusoidal voltage generating means includes a resonant circuit and means for dif,- ferentiating the impulse output of said trigger circuit to produce two pulses, the rst of said pulses corresponding to the leading edge of said impulse and the second of said pulses corresponding to the trailing edge of said impulse, the means for suppressing .the sinusoidal voltage being responsive to said rst pulse to damp out any oscillations present in said resonant circuit, and said resonant circuit being responsive to said second pulse to renew oscillations therein.

9. An electric pulse translator for translating the time modulated pulses of a given source into an equivalent train of time modulated impulses comprising an oscillating trigger circuit operating at a lower pulse frequency than the unmodulated pulse repetition rate of said given source for production of an impulse for each operating interval thereof, means responsive to the operation of said circuit to generate a sinusoidal voltage having a selectable frequency relationship with the unmodulated repetition krate of said pulses, means to apply the sinusoidal voltage to said circuit to increase the susceptibility thereof to actuation by input pulses, said frequency relationship being selected to time an undulation of said sinusoidal voltage so that the peak portion thereof covers an interval timed to substantially coincide with the maximum degree of modulation of the pulses from ,said source, and means responsive to the initiation of circuit operation to damp out said sinusoidal voltage so that the applied sinusoidal voltage Will be eliminated prior to the production of a new sinusoidal voltage in response to the next succeeding circuit operation.

10. An electric pulse translator for translating the time modulated pulses of a given source into an equivalent train of time modulated'impulses comprising an oscillatingtrigger circuit including a vacuum tube having plate, grid and cathode electrodes, means for applying pulses from said source to at least one of saidelectrodes, a gridcathode circuit, an impulse output circuit connected to at least one of said electrodes to pro-fl duce output impulses in response to oscillations established in said grid-cathode circuit, means responsive to current ow through said output circuit to generate a sinusoidal voltage having a selectable frequency relationship with therepetiftion rate of said pulses, means to apply the sinusoidal voltage to the grid electrode to control the susceptibility of the trigger circuit to actuation by input pulses, said frequency relationship being selected to time an undulation of said sinusoidal voltage so that the peak portion thereof covers an interval timed to the repetition occurrence, and at least as large as the maximum degree of modulation, of the pulses from said source, means for causing the initiation of the generation of said sinusoidal voltage upon termination of each cycle of trigger circuit operation, and means to suppress the sinusoidal voltage upon initiation of each circuit operation.

11. The translator dened in claim wherein the sinusoidal voltage generating means includes a resonant circuit connected in said output circuit and means for differentiating the impulse output of said trigger circuit to produce two pulses, the first pulse corresponding to the leading edge of said impulse and the second pulse corresponding to the trailing edge of said impulse, and the means for suppressing the sinusoidal voltage being responsive to the first pulse to damp out any oscillations present in said resonant circuit, and said resonant circuit being responsive to said second pulse to renew oscillations therein.

12. The translator defined in claim 10 wherein the sinusoidal voltage generating means includes a resonant circuit connected in circuit with the cathode electrode and means for differentiating the impulse output of said trigger circuit to produce two pulses the first pulse corresponding to the leading edge of said impulse and the second pulse corresponding to the trailing edge of said impulse, and the means for suppressing the sinusoidal voltage being responsive to said first pulse to damp out any oscillations present in said resonant circuit and said resonant circuit being responsive to said second pulse to renew oscillations therein.

13. The circuit defined in claim 10 wherein the sinusoidal voltage generating means includes a resonant circuit connected in circuit with the cathode electrode of the trigger circuit tube so that current of said output impulses flows through said resonant. circuit, and the means for suppressing the sinusoidal voltage includes a vacuum tube having the cathode and anode electrodes thereof connected across the resonant circuit, said vacuum tube having a control grid, and a battery connected between said control grid and the cathode electrode of said trigger circuit tube whereby said vacuum tube is normally biased to cut-off by said battery and rendered conductive by said output impulses.

14. In an electric pulse translator having an oscillating trigger circuit normally operating at a considerably lower pulse frequency than the unmodulated pulse repetition rate of a given pulse source so that the circuit is normally susceptible to operation by wanted and unwanted pulses only during the last portion of its normal operating period, the circuit when triggered operating to produce an impulse; means to generate a sinusoidal voltage having a selectable frequency relationship with the unmodulated repetition rate of said pulses, means to initiate the generation of said sinusoidal voltage upon termination of a circuit operation and means to suppress the sinusoidal voltage following initiation of circuit operation, and means for utilizing the sinusoidal voltage to increase the susceptibility of the circuit for the peak portion of a selected undulation of said sinusoidal voltage so that the circuit is highly 10 s susceptible during signalling intervals when desired signal pulses occur and much less susceptible during the time between said signalling intervals.

15. In an electric pulse translator having an oscillating trigger circuit operating ata lower pulse frequency than the pulse repetition rate of a given pulse source for production of an impulse for each operating interval of the circuit, said trigger circuit being actuatable by the pulses of said source and also by undesired pulses; the method of decreasing the possibility of operation in response to said undesired pulses and of increasing susceptibility of the circuit to operation in response to the pulses of said source comprising producing upon termination of each operating interval of said trigger circuit a sinusoidal voltage having a selectable frequency relationship with the repetition rate of said pulses, applying the sinusoidal Voltage to said trigger circuit to control the susceptibility thereof to actuation of input pulses, and selecting a frequency relationship for the sinusoidal voltage such that a selected un'dulation of said sinusoidal voltage is caused to occur so that the peak portion thereof coincides substantially with a pulse from said source, and suppressing the sinusoidal voltage upon the initiation of each operating interval.

16. In an electric pulse translator having an oscillating trigger circuit operating at a lower pulse frequency than the unmodulated pulse repetition rate of a given time modulation pulse source for production of an impulse for each operating interval thereof, said circuit being` actuatable by pulses of said source and by undesired pulses when such pulses produce in said circuit a voltageV exceeding a predetermined potential level; the method of decreasing the possibility of operation in response to said undesired pulses and of increasing susceptibility of said circuit to operation in response to the pulses of said source comprising producing upon termination of each operating interval of said trigger circuit a sinusoidal voltage of a frequency relationship with the unmodulated repetition rate of said pulses so that the peak portion of an undulation thereof is of sufficient width to cover a pulse reception interval for at least the maximum degree of modulation of the pulses of said source, applying the sinusoidal voltage to said oscillating circuit in a manner to increasethe susceptibility of the circuit during the desired pulse reception intervals and to maintain aV much lower susceptibility to input pulses duringthe time between the desired pulse reception intervals, and suppressing the sinusoidal voltage upon the initiation of each operating interval to prevent energy carry over from one sinusoidal voltage to the next.

DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number Name Date 2,277,000 Bingley Mar. 17, 1942 2,171,536 Bingley Sept. 5, 1939 2,061,734 Kell Nov. 24, 1936 2,113,214 Luck Apr. 5, 1938 FOREIGN PATENTS Number Country Date 416,126 British Sept. 13, 1934 

